Field of the Invention
Embodiments of the present application relate to the art of a gear assembly having a plurality of gears, and especially to a texture of tooth surface of the gear.
Discussion of the Related Art
JP-A-2004-308665 describes a gear having enhanced lubricating oil holding property between tooth faces of a hypoid pinion fitted onto drive shaft and a hypoid ring gear connected to a driven shaft. According to the teachings of JP-A-2004-308665, the skewness of the tooth flanks is set at −1 or less and the maximum height Ry is set at 3 or less so that Ry/Ra falls within 6-12. JP-A-2004-308817 describes a gear pair in which a tooth surface of one of gears is adjusted to as taught by JP-A-2004-308665. According to the teachings of JP-A-2004-308817, in the other gear, skewness is set at 0-1.5 and the maximum height Ry is set at 3 or less so that Ry/Ra is restricted to 6 or less.
PCT international publication WO 2004/108356 A1 describes a planetary gearbox comprising a hollow wheel gear, two or more planet gears, and a sun gear. In order to reduce metal debris generated by metal-to-metal contact during torque transmission, according to the teachings of WO 2004/108356 A1, tooth surface of each gear is superfinished to a surface roughness of 0.25 μm or
JP-A-2014-95392 describes a gear having an enhanced power transmission efficiency and strength of tooth face. According to the teachings of JP-A-2014-95392, in order to keep a proper thickness of an oil film between tooth surfaces, an arithmetic average roughness Ra of the tooth surface is restricted to 0.15 μm or less, and a height Rpk of a projection part is adjusted between 0.01 μm and 0.1 μm.
JP-A-2005-214339 describes a deceleration mechanism in which an engagement friction between a metal driving gear and a resin driven gear is reduced. According to the teachings of JP-A-2005-214339, an arithmetical average surface roughness Ra of gear teeth of the drive gear is adjusted less than 0.1 μm.
JP-A-2009-236244 describes a gear treatment method for extending a fatigue life of gear. According to the teachings of JP-A-2009-236244, thermal treatment is applied to at least one of tooth flanks of a first gear and a second gear engaged with each other, and then shot peening is applied to said one of tooth flanks. Preferably, the shot peening is applied to the tooth flank meshed more frequently than the other tooth flank or harder than the other tooth flank.
Thus, according to the foregoing prior art documents, surface texture of a tooth surface of gear is adjusted in such a manner as to reduce friction between gears. In the conventional tooth forming method, the tooth forming is carried out first of all and then the hardening treatment is applied to the tooth, and thereafter the surface treatment is applied to the tooth surface. That is, if the tooth surfaces of a plurality of gears are treated by the conventional tooth forming method, man-hour may be increased and production yield may be lowered. Consequently, production cost of gears and a gear unit may be increased.
In addition, in a powertrain of automobile, high-strength gears are used to transmit large torque from a prime mover to drive wheels. For this reason, the drive gear taught by JP-A-2005-214339 may not be suitable to be used in a powertrain of automobile.